Case in point: Dating "The Ball of Kirriemuir".The earliest extant text of "The Ball" is in Forbidden Fruit (later 19th century), a unique volume in the Murison Library, Dunfermline. In his introduction to The Merry Muses of Caledonia (1959 and later) the Burns expert James Barke refers to "The Ball of Kirriemuir" and relates its probable origin in an orgy occurring in north-east Scotland in the mid-1880s.
My late friend Gershon Legman however was categorical in his rejection of this.
In The Horn Book (1964) he says: "No date is given, but from a satirical poem at p. 60, on Annie Besant's birth-control agitation and methods (which first attracted attention in 1874, the poem being later [n.b.] reprinted in The Pearl, 1879-80, after the sensation of the Bradlaugh-Besant trial for Knowlton's Fruits of Philosophy), it may be dated as no earlier than about 1875." He goes on from there to assume that that terminus (a TPQ) is actually a TAQ, saying later unequivocally that the book is "c. 1875".   Armed with this date, he pours scorn on poor James Barke for postulating an origin for "The Ball of Kirriemuir" in the eighteen-eighties.
Then of course he examined the contents further, and had to say in his edition of MMC (1965, 288-9): "The only date occurring in the volume is 1824, at p. 44, but there are references, in the section on erotic riddles, p. 35-37, to the Queen (Victoria), Princess Alexandra, Gladstone, Annie Besant, Charles Stewart Parnell and Mrs. O'Shea, the last indicating a date somewhere in the 1890's, Parnell's political career having been ruined by the publicity given to Capt. O'Shea's divorce accusations in Nov. 1890." In spite of this Legman gives a "circa" date of 1890 to the volume, evidently wanting it to be as early as possible.         
So the date of Forbidden Fruit has to be at least 1891, and probably a year or two after that, and Barke may be more or less correct; it is possible that in five years or so [or ten!] a song composed on a memorable orgy could find its way to print, particularly since the texts, on linguistic grounds, seem to derive from the North-East.
It is a pity that Legman so forcefully "proved" Barke to be misinformed, since his opinion is cited by such as Bob Pegg in his analysis of "Rugby Songs" — see his Folk (London, 1976), p. 81, calling Barke's anecdote "folklore" (i.e. rumoured rubbish); and note that a decade after Legman recanted, his first opinion is still taken as valid.